This invention relates to the location of objects in a distributed computer system.
Computer programs are contained in a defined portion of memory known as the address space of the program. A local procedure call ("LPC") is a well known mechanism for the transfer of control and data within a single address space. A remote procedure call ("RPC")is an extension of this mechanism to provide for transfers between different address spaces.
FIG. 1 shows a distributed system having nodes 1, 2, 3 . . . N, the nodes connected by some communication link 10 (e.g., coaxial cable (e.g. Ethernet), fiber optic link, modem and telephone lines, or anything that connects two nodes such that they can communicate). Each node is defined as one address space. (It should be noted, however, that a node can be defined as more than one address space.) Each node contains one or more application objects such as A.sub.1 and B.sub.1 at node 1, A.sub.2 at node 2, etc. The nodes are typically supported by stable or non-volatile storage servers 1, 2 . . . M. Each node also includes an RPC system 12, is included in a host system (not shown) and is supported by standard services not shown in FIG. 1, such as run time support and communication services, etc.
Application objects often need to "invoke" other objects, or locate other objects, usually for the purpose of performing some operation on the objects. An object can be invoked without performing any operation on the object if, for example, it is merely necessary to determine the object's location. (This is an unusual case and most invocations will be for the purpose of performing an operation on the object.) Application object A.sub.1 can invoke B.sub.1 with an LPC since both objects are contained in the same node. However, A.sub.1 must use an RPC to access objects at other nodes, such as A.sub.2. When using an RPC, the original or calling object (also called the "client") must indicate the location of the node to which the RPC is to be directed A node that accepts an RPC is known as a "server." The client typically identifies the server and establishes the relationship between the client and server. To accomplish this, the client must pass an explicit or implicit argument to every RPC. This argument is a data structure, known as a binding, that includes the address of the server and possibly additional information such as the communication protocol and data representation on its host.
It is generally a goal of an RPC implementation to make the semantics of an RPC as close as possible to those of an equivalent LPC. This enables programmers to write distributed applications (i.e., applications distributed among two or more nodes) without having to be aware of or take into account network protocols or external data representations.
Locating the proper server (i.e., locating invoked objects) is sometimes difficult and is a burden on the programmer. Several methods have been developed to reduce the burden of the binding step on the programmer by automating the location of the server to at least some extent.
One method is known as default binding, where the RPC system chooses the server, either non-deterministically, or in a way that depends on factors that are normally hidden from the client, such as network distance.
Location independent invocation ("LII")is a method of completely relieving the programmer of the burden of choosing the server. In other words, the programmer can treat an RPC in the same way as an LPC. LII can be implemented at the application level, in which an application dependent sub-routine package uses its knowledge of the call semantics to choose the server For example, in an environment in which a file name indirectly identifies the file server that stores the file, a file system "clerk" might be used to direct an RPC to the appropriate server.
In large distributed systems it is desirable to permit application objects to move between address spaces for a variety of reasons. For example, mobile objects increase availability and reconfigurability, will allow special capabilities of a particular machine to be utilized by moving the object to that machine, and may reduce communication costs. In addition, object mobility provides a convenient way for application data to be moved to the point where it is needed without the necessity of making a copy and keeping it consistent with the original
As an example of such a mobile object, consider an expense voucher circulating within a corporation over a large distributed network having many nodes (each node potentially corresponding to a user station). A form is filled in by an employee and electronically signed by different managers who approve or reject the expense reimbursement represented by the form. If appropriate, the accounting department will make a petty cash disbursement and then the form may be archived for tax and audit purposes. The people acting on the form may all be located in one building or may be at different locations throughout the world. Storing these forms at a central location is impractical since each person working on the form desires a short interactive response time. Sending commands over great distances to a central storage area would result in a slow response time and high communication costs. Therefore, it is desirable to move the object to the node most convenient to the current user.
Finding the proper server node becomes increasingly difficult when invoking mobile application objects. One solution is to inform all instances of the distributed application program of each move and of the new address of the object, so that, e.g., each can record the new location of the object.
Systems have also been developed that use "forwarding addresses" to locate objects that have moved. If an object moves from a first node to a second node, the first node stores a forwarding address that includes the location of the second node and the time that the object moved. Each forwarding address includes a time component so that if two forwarding addresses are obtained for the same object, the system can select the newer address and thereby obtain more current information on the object's location. The time can be real time or it can be given by the number of moves that an object has made. If one forwarding address has a "time" of 3 moves and another forwarding address for the same object has a "time" of 4 moves, then the second address must be more current than the first since the object's address after 4 moves must be more recent (or younger) than its address after three moves.
If the object moves several times, each old node will have the address of the next node to which the object moved thereby creating a "chain" that can be followed to locate the object.